VMM/config: Updated Dovecot version check. Renamed function check_version_format() -> check_dovecot_version(). Now this function checks also if misc.dovecot_version >= MIN_DOVECOT_VERSION.

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Upgrading vmm
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Upgrade instructions for different versions of vmm.

.. toctree::
   :maxdepth: 1

   upgrade/0.5-0.6
   upgrade/0.4.x-0.5